Obtains a reference to the inner allocator used to declare this scoped_allocator_adaptor.

If sizeof...(InnerAllocs)==0, that is, no inner allocators were declared, returns *this. Otherwise returns a reference to std::scoped_allocator_adaptor<InnerAllocs...>, that is, a scoped allocator composed of all inner allocators of *this, with the first inner allocator becoming the outer allocator.